Enable sigaltstack and dtrace on FreeBSD

Enable the use of sigaltstack and DTrace on FreeBSD. Note that while DTrace is enabled here (with `has_dtrace=yes`), the DTrace hooks will only be compiled into Mono when the FreeBSD kernel has been compiled with DTrace support.
Merge pull request #679 from jack-pappas/patch-6

Enable sigaltstack and dtrace on FreeBSD